Cellular substrates of iron overload cardiomyopathies
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Date
25/11/2011Item status
Restricted AccessEmbargo end date
31/12/2100Author
Baptista-Hon, Daniel Tomas
Metadata
Abstract
Cardiomyopathies and arrhythmias are major causes of death in untreated hereditary
haemochromatosis, acute iron poisoning and during secondary iron overload
resulting from repeated blood transfusions in β-thalassaemia. Iron overload
cardiomyopathies are associated with systolic and diastolic dysfunction, suggesting
that Ca2+ homeostasis is impaired. However, the cellular mechanisms of these
dysfunctions are unknown. The data presented in this thesis establishes for the first
time iron effects on cardiomyocyte Ca2+ handling, as well as the potential cellular
substrates responsible for this impairment during iron overload.
Exposure of isolated rat ventricular cardiomyocytes to 200μM iron led to biphasic
changes in systolic Ca2+ release. Phase 1: an initial reduction of systolic Ca2+ release
followed by; Phase 2: increased Ca2+ release with arrhythmogenic spontaneous Ca2+
release, cell contracture and cell death. There is evidence that Fe2+ enters
cardiomyocytes via L-type Ca2+ channels (LTCC) and reduces the Ca2+ trigger. The
close apposition of LTCCs to cardiac ryanodine receptors (RyR2) suggests RyR2
may be a first target. Indeed RyR2 activity was drastically reduced on exposure to
nanomolar [Fe2+] in single channel studies. Together with evidence that Fe2+ may
reduce the Ca2+ trigger from LTCC, this is consistent with iron reducing
sarcoplasmic reticulum (SR) Ca2+ release during Phase 1. In Phase 2, the presence of
spontaneous Ca2+ release events is consistent with SR Ca2+ overload. Indeed, in
single rat ventricular cardiomyocytes SR Ca2+ content was found to be increased by
27% during Phase 2. The cellular substrates responsible for this increased SR Ca2+
content were 2-fold: 1) through reduced extrusion via both the Na+ Ca2+ Exchanger
(NCX) and Plasmalemmal Ca2+ ATPase (PMCA) and 2) through increased
resequestration via the SR Ca2+ ATPase.
Iron catalyses the production of reactive oxygen species (ROS) during the Fenton
reaction. To investigate whether iron effects might be due to ROS, I used the cell
permeant ROS scavenger Tempol. Tempol attenuated Phase 2 effects but Phase 1
effects were not affected. This is consistent with the hypothesis that Phase 1 effects
were due to direct effects of Fe2+ affecting LTCC trigger and RyR2 function. The
attenuation of Phase 2 effects suggests that ROS damage to key Ca2+ handling
mechanisms, such as NCX and PMCA might account for a reduced Ca2+ extrusion
and subsequent SR Ca2+ overload.